1. Field of the Invention
The present invention relates to elastic-wave filter devices, and specifically, an elastic-wave filter device that includes a longitudinally coupled resonator-type elastic-wave filter.
2. Description of the Related Art
An elastic-wave filter device in which an elastic-wave filter that utilizes an elastic wave, such as a surface acoustic wave propagating along the surface of a piezoelectric substrate or a boundary acoustic wave propagating along the boundary between a piezoelectric substrate and a solid layer, accommodated in a package, has been proposed. For such an elastic-wave filter device, the degree of out-of-band suppression may be decreased by a parasitic component of the package.
To improve out-of-band suppression, for example, Japanese Unexamined Patent Application Publication No. 11-145772 discloses, as a package on which a surface-acoustic-wave filter is mounted surface-down, a configuration illustrated in the plan view of FIG. 19.
That is, as illustrated in FIG. 19, a filter mounting pattern provided on a filter mounting surface of a package substrate 100 is electrically connected to input and output electrodes and a ground electrode of a surface-acoustic-wave filter by conductive bumps and connection patterns 141; 142a; 142b; 143a to 143d; 144a, 144b. Among the pattern elements of the filter mounting pattern, an input ground pattern element 133 and an output ground pattern element 134 are separated from one another. The pattern elements 131, 132a, 132b, 133, and 134 are electrically connected to external connection terminals 121, 122a, 122b, 123a to 123d, and 124, respectively, which are provided on side surfaces of the package substrate 100 in order to connect to an external circuit. An example of this configuration is described as having been able to improve out-of-band attenuation in a higher range than the pass band by separating the ground pattern elements.
However, with the advancement of miniaturization of products, directly connecting a plurality of ground pattern elements to a plurality of external connection terminals provided on side surfaces of the package substrate, as illustrated in FIG. 19, becomes impossible. Because of this, the pattern elements provided on the filter mounting surface of the package substrate are connected to the external connection terminals provided on the back side of the package with via conductors disposed therebetween. In this case, when the ground pattern elements are separated from one another, the area of each of the ground pattern elements is relatively small, such that the number of via conductors that can be arranged in the ground pattern elements is relatively few. This results in an increased potential difference between the potential of the ground pattern element and the reference potential surface, which leads to an increase in high-frequency current produced as a result of a parasitic component. Therefore, the degree of out-of-band suppression is reduced.